A Standard Candle in the Dark

Show notes for Episode 192, April 7, 2014Hosts: Paul, Rob C., JesseTitle: A Standard Candle in the Dark
This week, we traversed the Solar System discussing a new way to age our own Moon, checking in on the “swimming pool” suspected by Cassini gravity measurements to exist beneath the south pole of Enceladus (satellite of Saturn) and finally meandering out to the realm of the Trans Neptunian Objects. We then set sail for the gamma ray signatures in our own galaxy (dark matter anyone) before charging off to the distant shores of AGNs and the possible (and exciting) new standard candle method involving accretion disks and dust disks around massive central black holes.

This week in space/astronomy history:1. April 9, 1959: NASA selects the original seven Mercury astronauts2. April 12, 1961: Yuri Gagarin is the first human in space and to orbit the Earth and thus the celebration of Yuri’s Night.
3. April 7, 1972: Birth of British astronaut Tim Peake.
4. April 11, 1986: Halley’s Comet makes it’s closest approach to Earth, inspiring millions.
5. April 7, 2001: Launch of Mars Odyssey.

News:1. Two more! Sedna, an object three times further from the Sun than Neptune, was discovered in 2003 (Sedna perihelion: 76 AU; 1000-km diameter). On March 26 earlier this year, another slightly more distant object was also discovered: 2012VP113 (perihelion: 80 AU; half the diameter of Sedna). The same team that discovered 2012VP113 have announced the discovery of two more objects: 2013FY27 and 2013FZ27. All three objects were discovered with the 470 megapixel Dark Energy Camera on the 4-m Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO). Approximations for FY27 are currently 50 AU distant and 600 km diameter, while FZ27 are about 80 AU and 1000 km. Our Solar System could be about to get a lot more crowded – or rather, we actually realize that it is! Suggested reading: io9 article, New Scientist2. Hidden ocean on Enceladus: Cassini’s 2005 imaging of Enceladus’ geysers have been the subject of ongoing analysis. The geysers could simply have been caused by water being squeezed out from the moon as different ice layers move past one another. But while direct evidence of subsurface oceans remains elusive, the gravitational field reveals that the material below the ice where those geysers appeared is more dense than ice. This suggests a large body of water, and the possible source of the geysers. Suggested reading: io9 article, Science@NASA article, Science3. Lunar eclipse next week: Reminder of the total lunar eclipse next week. Suggested reading: wiki page4. Another step closer with evidence for dark matter: A gamma-ray map of our Galaxy, made using data from NASA’s Fermi Gamma-ray Space Telescope, reveals a number of gamma-ray sources. When all known sources are accounted for, and subtracted from the map, a central overdensity of gamma-rays is observed. This overdensity matches the profile that would be expected if dark matter particles were annihilating with one another, and emitting gamma-rays in the process. The researchers in this study remain cautious that there could be other as-yet unidentified “normal” sources of gamma-rays contributing to this signal, but this work is an exciting step forward for dark matter detection. Suggested reading: NASA5. Accurately determining the age of the Moon: The Moon is thought to have formed about 4.5 billion years ago after a Mars-sized object collided with a proto-Earth, resurfacing the Earth in a cataclysmic event that also led to the creation of the Moon. But exactly when the Moon formed is still not certain. Previous estimates were between 30 and 100 million years after the formation of the Solar System. New estimates put this towards the younger end (95 +/- 30 million years); this is still a large range, but rules out the older time period. Rather than use radioactive dating, which can a large spread, this new study took samples of the Earth’s composition combined with computer simulations. The basic idea is that once the Earth became molten again during the Moon-forming impact, all the iron sank to the core (because it is more dense). Any iron on the surface that we find now must have come from subsequent impacts. Suggested reading: Nature article6. NASA-Russia split – except for ISS: It costs the US $70 million for every astronaut sent up in a Soyuz rocket. Suggested reading: NASA’s statement, The Verge (NASA’s internal memo).

Major Topics Discussed:A new galactic “standard candle”. Yoshii et al have proposed that an examination of the black hole accretion disks at the centres of AGN (Active Galactic Nuclei) galaxies may give astronomers a new standard candle for measuring distant galaxies. Trigonometric parallax, Cepheid variables and supernovae type 1a are all known for their ability to accurately determine distances. The snag of course is that with increasing distance, all these methods begin to “fail” leaving more indirect methods for determining distance.
Using data over 6 years from the 2-meter MAGNUM telescope at the Haleakala Observatory in Maui, Hawaii, the team has concluded that H_0 is around 73 +/- 3 km/sec/MPC, a value in keeping with most accepted values of the Hubble constant EXCEPT for the value determined by analysis of the CMB (Cosmic Background Radiation) by the Planck spacecraft (H_0 ~ 68 km/sec/MPC). (Hubbles Law, a plot of distance versus recessional speed, is now extended out to over 500 million light years!)
The basis for the new standard candle is to assume that a dust ring forms around the outside of the accretion disk that surrounds the galaxy’s central black hole. The dust ring cannot be too close to the accretion disk because of its heat (dust grains cannot form when too hot). Once formed the grains will be heated by the accretion disk though and the distance (gap) between the dust ring and the accretion disk will be a measure of the intrinsic brightness of the AGN itself. Thus distance can be determined. How do you measure the gap? By observing the UV light from the accretion disk and look for the time lagged infrared emission from the dust ring.
Obviously the model is heavily dependent upon the model for the dust ring (and must be corrected for matters such as redshift, dust absorption, etc) but the authors are confident that the model is sound and has been compared for nearby galaxies with distance values found from Cepheid variables. Bottom line: you can never have too many independent methods to measure galactic distances. Suggested reading: http://arxiv.org/pdf/1403.1693v1.pdfHere’s an one of the original papers on the subject.
Watson et al. 2011, A new cosmological distance measure using AGN (how to use reverberation mapping of AGN to probe dark energy at redshifts z>2) http://lanl.arxiv.org/abs/1109.4632